Common Quality Defects Of FAG Spherical Bearing Parts After Heat Treatment

Common quality defects of FAG bearing parts after heat treatment include: overheating, underheating, quenching cracks, insufficient hardness, heat treatment deformation, surface decarburization, and soft spots in the quenching microstructure.

1. Overheating
   The overheating of the microstructure after quenching can be observed from the rough mouth of FAG bearing parts.
   However, to correctly judge the degree of overheating, the microstructure must be observed. If coarse needle-shaped martensite appears in the quenching structure of gcr15 steel, it is a quenching overheating structure. The cause of the formation may be comprehensive overheating caused by excessive quenching heating temperature or too long heating temperature time; it may also be due to the serious banded carbides in the original structure, forming local martensite needles in the low-carbon zone between the two bands, causing local overheating. The amount of residual austenite in the overheated structure increases, and the dimensional stability decreases. Due to the overheating of the quenching structure, the crystals of the steel are coarse, which will lead to a decrease in the toughness of the parts, a decrease in the impact resistance, and a decrease in the life of the spherical bearing. Severe overheating may even cause quenching cracks.
2. Underheating
   If the quenching temperature is too low or the cooling is poor, the troostite structure exceeding the standard will be produced in the microstructure, which is called underheating structure. It reduces the hardness and wear resistance sharply, affecting the life of the joint bearing.
3. Quenching cracks
   The cracks formed by the internal stress of FAG bearing parts during the quenching and cooling process are called quenching cracks. The reasons for this crack are: due to the quenching heating temperature being too high or the cooling being too rapid. The thermal stress and the organizational stress during the change of metal mass volume are greater than the fracture strength of the steel; the original defects of the working surface (such as fine cracks or scratches on the surface) or the internal defects of the steel (such as slag inclusions, serious non-metallic inclusions, white spots, shrinkage cavity residues, etc.) form stress concentration during quenching; severe surface decarburization and carbide segregation; insufficient firepower after quenching or timely tempering of parts; excessive cold stamping stress caused by the previous process, forging folding, deep turning tool marks, sharp edges and corners of oil hooks, etc.
4. Heat treatment deformation
   During heat treatment, FAG bearing parts have thermal stress and structural stress. This internal stress can be superimposed or partially offset each other. It is complex and changeable because it can change with the changes in heating temperature, heating speed, cooling method, cooling speed, part shape and size, so heat treatment deformation is inevitable. Understanding and mastering the law of change can put the deformation of joint bearing parts (such as the ellipse of the sleeve, the size expansion, etc.) within a controllable range, which is conducive to the production. Of course, mechanical collision during the heat treatment process will also cause the parts to deform, but this deformation can be reduced and avoided by improving the operation.
5. Surface decarburization
   During the heat treatment process of FAG bearing parts, if they are heated in an oxidizing medium, oxidation will occur on the surface, reducing the mass fraction of carbon on the surface of the parts, causing surface decarburization. If the depth of the surface decarburization layer exceeds the final processing allowance, the parts will be scrapped. The depth of the surface decarburization layer can be determined by metallographic method and microhardness method in metallographic inspection. The surface layer microhardness publication curve measurement method is accurate and can be used as an arbitration criterion.
6. Soft spot
   The phenomenon of insufficient local hardness on the surface of spherical bearing parts due to insufficient heating, poor cooling, improper quenching operation, etc. is called quenching soft spot. It can cause a serious decrease in surface wear resistance and fatigue like surface decarburization.